Disturbed Ca²⁺ kinetics in N-deacetylase/N-sulfotransferase-1 defective myotubes

The biosynthesis of heparan sulfate, present on the cell surface and in the basal lamina surrounding cells, is a multistep process in which each step is mediated by a specific enzyme. The initial modification of the precursor polysaccharide, N-deacetylation followed by N-sulfation of selected N-acetyl-D-glucosamine residues, is catalyzed by the enzyme glucosaminyl N-deacetylase/N-sulfotransferase (NDST). This event is a key step that regulates the overall sulfate content of the polysaccharide. Here, we report on the effects of NDST deficiency on Ca²⁺ kinetics in myotubes from NDST-1- and NDST-2-deficient mice, indicating a novel role for heparan sulfate in skeletal muscle physiology.

Immunostaining for specific heparan sulfate epitopes showed major changes in the heparan sulfate composition in skeletal muscle tissue derived from NDST-1^-/- mice and NDST^-/- cultured myotubes. Biochemical analysis indicates a relative decrease in both N-sulfation and 2-O-sulfation of skeletal muscle heparan sulfate. The core protein of heparan sulfate proteoglycan perlecan was not affected, as judged by immunohistochemistry. Also, acetylcholine receptor clustering and the occurrence of other ion channels involved in excitation-contraction coupling were not altered. In NDST-2^-/- mice and heterozygous mice no changes in heparan sulfate composition were observed. Using high-speed UV confocal laser scanning microscopy, aberrant Ca²⁺ kinetics were observed in NDST-1^-/- myotubes, but not in NDST-2^-/- or heterozygous myotubes. Electrically induced Ca²⁺ spikes had significantly lower amplitudes, and a reduced removal rate of cytosolic Ca²⁺, indicating the importance of heparan sulfate in muscle Ca²⁺ kinetics.